1. Field of the Invention
The present invention relates to a steel tower, and more particularly, to a power transmission tower.
2. Description of Related Art
A power transmission tower refers generally to a tower made of steel frame or steel reinforced frame for supporting power transmission lines. In the power transmission tower, a tower body of metal material is in contact with the earth. In order to insulate the power transmission line from the earth, an arm is used to maintain a clearance between the power transmission line and the tower body. Since the arm is made of a metal material, an insulator of porcelain material is used to insulate the arm from the power transmission line.
FIG. 1 shows a conventional tower for two coupled three-phase power transmission. Referring to FIG. 1, in a conventional tower for two coupled three-phase power transmission, arms 2 of metal material for supporting two coupled three-phase power transmission lines 4 are installed to the tower body 1. The arms 2 are disposed in the tower body 1 in consideration of ensuring of clearances between the power transmission line 4 and the tower body 1 and between the power transmission lines 4. To insulate the arm 2 from the power transmission line 4, an insulator 3 is installed at a tip of the arm 2 and the power transmission line 4 is hung on the tip of the insulator 3. On an upper end of the tower body 1, an arm 5 for supporting an overhead earth wire 6 is installed.
The conventional tower for the two coupled three-phase power transmission has an enlarged structure for supporting tension of the power transmission line since it is mainly used to cross over a valley or a river having a long span. Therefore, it is difficult to install the conventional power transmission tower in a place such as a downtown area where an area of a land under the power transmission is narrow.
To miniaturize the power transmission tower is to replace the arm of metal material with an arm of polymer material having high insulation function. As a polymer composite material having an insulation function exceeding a mechanical strength of metal is developed with development of polymer material and forming technology, a composite insulator using Fiber glass Reinforced Plastic (FRP) has become able to replace the arm of metal material.
FIG. 2 shows a conventional delta shaped compact tower. Referring to FIG. 2, in a conventional delta shaped compact tower, main insulation arms 9 for supporting two coupled three-phase power transmission lines 11 are installed to the tower body 8. Supporting forces of the main insulation arms 9 is reinforced by auxiliary insulation arm 10 connected to the tower body 8 in correspondence to the main insulation arms 9 respectively. A pair of the main insulation arm 9 and the auxiliary arm 10 forms a delta shape. On an upper end of the tower body 8, an arm 13 for supporting an overhead earth wire 12 is installed.
The conventional delta shaped compact tower is advantageous in miniaturization and light weight of the tower since it is possible to reduce the clearance between the arm and the tower body and lower a height of the tower by the length of the insulator installed on the arm of metal material as compared to the case of using the arm of metal material, but requires reinforcement of the supporting force on the main insulation arm and improvement in electrical properties of the tower itself.